In large endless non-metallic conveyor belts, which may have metal reinforcing cables therein, used to convey bulk material, there is a possibility of encountering a rip in the belt, for example, by a sharp object dropped thereon at the loading station. It is desirable promptly to detect such rips and, preferably, to shut down the conveyor belt upon such detection, thereby minimizing damage to the belt. One such conveyor belt rip detector is disclosed in U.S. Pat. No. 3,792,459. In such rip detector plural antennas, which may be single electrical conductors, are embedded in the belt transversely to its length at spaced-apart locations in the belt. An electrical signal is coupled by respective antennas from a transmitter to a receiver as the belt moves and the respective antennas pass in capacitive coupling relation with the transmitter and receiver at a rip detector station, and the receiver thus delivers an input signal to detector circuitry which interprets the same as an indication of satisfactory belt integrity. However, a broken antenna, for example at a place where the belt has been ripped, will not couple the transmitter signal through to the receiver, and the detector then senses the same as an indication of the occurrence of a rip condition. The detector circuit in such patent is operative after the lapse of a predetermined time period corresponding to the passage of a given number of broken antennas past the rip detector station to produce a distinguishable output that activates an alarm and/or deactivates the conveyor belt drive.
A number of improvements for use in conveyor belt rip detectors are disclosed in U.S. Pat. No. 4,228,513. One of such improvements includes a means for detecting the progress of the conveyor belt to know when an antenna should be at the rip detector station. If there is no antenna present then, a prompt shutdown of the conveyor belt drive may be effected.
Wear, stretching, contraction, dirt, other environmental conditions, etc. may cause a variation in the efficiency of signal coupling, whether of the capacitive, inductive, optical, or any other type of coupling, between the antennas (or other signal coupling means carried by the belt) and the transmitter and the receiver at a rip detector station. Such efficiency variation will vary the magnitude or other parameter of the input signal delivered from the receiver to the detector, which may detrimentally affect operation of the entire system.
Thus, it would be desirable to provide signal information to the detector at a relatively accurately controlled magnitude (or other parameter). In copending, commonly assigned U.S. patent application Ser. No. 126,218, filed Mar. 3, 1980, now U.S. Pat. No. 4,349,883, issued Sept. 14, 1982, such magnitude control is effected digitally, step-wise and efficiently. An improved antenna detection technique and provisions to avoid nuisance shut downs of the conveyor belt also is disclosed in commonly assigned U.S. patent application Ser. No. 133,595 filed Mar. 24, 1980.
It has been discovered, as has been described in commonly assigned copending U.S. patent application Ser. No. 275,969, filed June 22, 1981, for "Signal to Noise Ratio Improvement for Rip Detectors" that in some operational environments of conveyor belt rip detectors the electrical noise is so great that the poor signal to noise ratio significantly decreases the accuracy of the rip detector function. Moreover, and very importantly, it has been discovered that in a conveyor belt that carries antennas intended for capacitive coupling with the transmitter and receiver at a rip detector station, much of the electrical noise introduced to the receiver is derived from cross coupling, primarily of a capacitive nature, between the capacitive probes of the transmitter and receiver via the belt itself and/or apparatus associated therewith, such as the rollers, drive wheels, support frame, etc. In connection with such discovery, an equivalent electric circuit model of the transmitter/belt/receiver connection has been developed and used to determine an efficient technique for improving signal to noise ratio even in extremely electrically noisy environments. In accordance with the invention of such application, and in opposite contrast to what ordinarily would be expected, the signal to noise ratio is improved by reducing the frequency of the AC signal ordinarily used in a rip detector system in which the transmitter and receiver are capacitively coupled to antennas sequentially carried past the rip detector station by a conveyor belt. The equivalent circuit model has been found to include between the two input and output probe capacitors a finite resistance of the antenna itself and a cross coupling capacitance operating in parallel with such antenna resistance.
In each of the above patents and applications diode detectors are used in the receiver to convert the modulated AC transmitter signal to DC pulses representing passage of integral antennas past the rip detector station. Such diode detectors, even with associated conventional filters, may be unsatisfactory to distinguish the antenna pulses from the noise in particularly electrically noisy environments.
The magnitude of the electrical noise signal in a conveyor belt rip detector often approaches the magnitude of the transmitter signal; and, therefore, it is quite difficult to sense a received transmitter signal with accuracy, especially in particularly electrically noisy environments in which conveyor belt rip detectors often are found.
The electrical noise problem becomes all the more acute in rip detector systems because there usually must be at least one or two signal couplings by non-contacting means, for example, capacitive or inductive coupling. (The invention will be described in detail below with respect to use of a capacitive coupling technique, vis-a-vis the coupling of the AC transmitter signal to an antenna carried by a conveyor belt and coupling of the transmitter signal from the antenna to the receiver. However, it is believed that the features of the invention may be used when other types of rip detector signal couplings are employed, such as the noted inductive coupling.)
In the typical prior rip detector, the periodic coupling and non-coupling of an AC carrier signal (the AC transmitter signal), is a function of whether or not an antenna is present at the rip detector station; and such periodic coupling may be considered modulation of the transmitter AC carrier signal. The poor signal to noise ratio often occurring, however, may make it difficult to discern whether or not a rip has occurred, and this, of course, would reduce the effectiveness of the rip detector.
One technique used for noise filtering or signal to noise ratio improvement is known as the lock in amplifier, a disclosure of which is presented in "Optimization of Electronic Measurements" by Howard Malmstadt, et al. (W. A. Benjamin Publishers, Menlo Park, Calif., at pages 118-125. Prior use of lock in amplifiers, however, has been generally restricted to laboratory environments, such as with the spectrophotometric measuring equipment in the Malmstadt publication. Moreover, to use effectively a lock in amplifier, it is necessary that a modulated AC carrier signal and a reference AC signal be substantially in phase with each other to achieve desired synchronous demodulation. Therefore, as is described by Malstadt, phase adjustment may be provided for the reference AC signal.
In commonly assigned, concurrently filed U.S. patent application Ser. No. 288,883 for "Improved Demodulation Technique for Rip Detector Signals" there is disclosed a lock in amplifier technique that enables synchronous demodulation of signals occurring in a rip detector. In that application is disclosed the discovery that the received AC transmitter signal, i.e. the AC transmitter signal received by the receiver or detector-amplifier from an antenna, may be shifted in phase from the transmitted AC transmitter signal, i.e. the AC transmitter signal transmitted by the transmitter for coupling to an antenna. That phase shift may be variable in a single conveyor belt rip detector system and also may vary from system to system. To use such lock in amplifier demodulation, then, it is necessary to shift the phase of the demodulating or reference signal.
The entire disclosures of the above mentioned patent, applications and reference are hereby incorporated by reference.